The Walk The Line star appeared disheveled and uncomfortable as he attempted to promote what he claimed would be his final film, Two Lovers, on the show in 2009. He evoked the presenter's wrath by refusing to remove his sunglasses for the studio interview and chewing gum as he spoke. Phoenix's brother-in-law, director Casey Affleck, has since admitted the appearance was part of an elaborate "social experiment" stunt he had been filming for his new documentary I'm Still Here, about the actor's spoof efforts to become a rapper.

The Oscar winner returned to Letterman's sofa on Wednesday, suited up and clean-shaven, and offered his sincere apologies for making a mockery of the host during his worrying 2009 interview. He told Letterman, "Thank you for having me on the show last time. We wanted to make this film. It was both our ideas. We wanted to do a film that explored celebrity between the media and the consumers and the celebrities themselves, and we wanted something that would feel really authentic."

Phoenix confessed he'd come up with the idea for the documentary after watching a slew of reality shows - because he was baffled as to how viewers could be so easily manipulated into thinking the TV stars were not just acting. He confirmed Letterman had no idea the previous interview had been a hoax and insisted there had been no script, following allegations the presenter had been in on the joke the whole time.

Phoenix said, "You've interviewed many, many people and I assumed you'd know the difference between a character and a real person but I apologize, I hope I didn't offend you in any way. We'd hoped to come on a talk show and I was looking for a beat down, and I got one. Thank you for that." The Hollywood star admitted he was stunned by how easily everyone was fooled - because some of his actions were so outrageous, Phoenix and Affleck were convinced people would see through the whole "experiment".

He continued, "To be frank, we didn't really know how the media or the public was going to respond. I mean, when I first announced that I was retiring - which in itself, I thought was a joke, you know a 35 year old who says he's retiring from acting - but I didn't know that anyone would care and frankly I assumed they wouldn't, but it kind of snowballed and became this big thing and then I came on the show here and that got a lot of attention."

At the end of his bizarre 2009 interview, Phoenix was filmed taking off his sunglasses and shaking Letterman's hand - and the actor reveals removing his disguise for the brief encounter landed him in trouble with Affleck: "I got in a lot of trouble for that with Casey. But I was eager to thank you for the opportunity."

But Phoenix insists he would do it all again - because the experience resulted in a very unique film. He added, "Yeah, 100 per cent. I don't regret it at all. The experience was incredible. I wanted to make a film in a different way and... the experience was very gratifying."